This invention relates to a foldable knife or tool and more particularly to a mechanism for automatically locking a tool or knife blade into extended position with respect to a handle.
One of the inherent problems with folding, tools is the tendency of the tool blade to fold back toward the handle during use. Aside from interrupting the work, a collapsing blade can damage the workpiece or injure the user.
A wide variety of locking mechanisms have been devised to overcome this problem. At one end of the spectrum are the mechanisms which require additional manipulation to lock the tool blade into extended position, and further manipulations to unlock the tool blade so that it may be refolded. While these types of locking mechanisms can be very effective, the additional steps required to lock and unlock the blade may be considered undesirable. Mechanisms of this type are disclosed in Leatherman U.S. Pat. No. 4,888,869, and German Patent No. 822,507.
At the other end of the spectrum are automatic locking mechanisms which require no additional manipulation beyond extending and refolding the blade. The major disadvantage of these types of locking mechanisms is that they may not be reliable to lock the blade in extended position. An example of such an automatic locking mechanism is shown in Leatherman U.S. Pat. No. 4,238,862, disclosing a flat portion on the base of a blade interacting with a spring. This type of locking mechanism is sometimes called a "slip back" lock.
In the middle of the spectrum are locking mechanisms which lock automatically, but require additional manipulation to unlock. Examples of these types of mechanisms are shown in French Patent No. 83,10567; Hart et al. U.S. Pat. No. 4,302,877; Leatherman U.S. Pat. No. 4,238,862; and Neely U.S. Pat. No. 5,060,379.
While the locking mechanism disclosed, for example, in FIG. 6 of Leatherman U.S. Pat. No. 4,238,862 is functional, it is not completely satisfactory. One problem is that a lip defining one side of a notch receiving a spring-loaded catch can act as a cam and force the catch out of the notch, releasing the blade from a desired locked position.
In folding knives or multi-blade tools, the base of each blade or tool bit typically has a projection, sometimes called a "kick," which projects a small distance beyond the shank of a tool bit or a sharpened edge portion of a blade, to rest on structure within the handle or other part of the knife which holds the bits or blades when they are folded. The kick acts as a limit stop to hold the tools or blades in desired positions and prevent a sharpened edge from contacting any surfaces inside the handle when the blades are in their folded configuration. For a tool such as that shown in Leatherman U.S. Pat. No. 4,238,862, it has been necessary in the past to remove material so as to bevel an outside face of the kick of each blade or tool bit located adjacent a sidewall portion of a handle in the form of a U-shaped channel, to avoid undesired sideward forces on the blade and allow the kick to rest on the inside of the base of the U shape of the channel. Forming such a bevel is a time-consuming additional operation which adds to the cost of such folding-blade tools.
What is needed, then, is an improved locking mechanism which is engaged automatically upon placing a blade in a desired position and which is less susceptible than previously known locking mechanisms to being disengaged by the very forces it is intended to resist in a multi-blade tool which can be manufactured economically.